Steam generator for cooking apparatus

ABSTRACT

According to the invention, a steam generator is provided with a housing for forming a steam generating chamber, the housing having a steam outlet port. A heater plate is supported by the housing within the steam generating chamber. A water inlet valve is fluidly connected to the housing for selectively supplying water into the steam generating chamber whereby water is directed to the heater plate for conversion to steam. The heater plate forms a bottom wall of the steam generating chamber and is arranged to have a high point. The water inlet valve is opened and closed in accord with a duty cycle which alternates between two phases: a fill phase duty cycle and a steady state duty cycle.

The present invention relates to a steam generating mechanism for use ina cooking application and more particularly to a steam generator for usewith a cooking apparatus, such as a cooking sink, microwave oven orconventional oven, which can provide steam cooking capability.

BACKGROUND OF THE INVENTION

It is sometimes preferable for particular kinds of foods to be cookedusing steam. Accordingly, conventional ovens and microwaves havesometimes been equipped with a steam generating mechanism in addition toa regular heating means. Additionally, it is known to provide sinks withcooking capabilities including having a steam generator which providessteam into the cooking sink.

Conventionally there has been introduced a wide variety of steamgenerating mechanisms into the market, which can be characterized intodifferent categories.

One type of steam generating mechanism which is typically used in anoven, has a means for providing water into the bottom portion of an ovencavity. An oven heater is installed into the bottom of the oven cavityand operates to heat the water so as to generate steam into the chamber.

Another type of steam generating mechanism employs a configuration inwhich the water is injected over a heater to generate steam. U.S. Pat.No. 6,318,246 illustrates such a system, wherein a water supply tubedirects water toward a suction side of a fan where air and water issucked by the fan and is dispersed toward a heater so that water isformed into steam.

U.S. Pat. No. 4,741,261 discloses another type of steam generatingmechanism. In this reference, a steam generator is arranged outside of acooking pot and is connected to the pot so that steam generated by thesteam supply can be directed into the cooking pot. A valve assemblybetween the steam generator and the cooking pot controls theintroduction of steam into the pot.

SUMMARY OF THE INVENTION

According to the invention, a steam generator is provided having ahousing for forming a steam generating chamber, the housing having asteam outlet port. A heater plate is supported by the housing within thesteam generating chamber or cavity. A water inlet valve is fluidlyconnected to the housing for selectively supplying water into the steamgenerating chamber whereby water is directed to the heater plate forconversion to steam. The heater plate forms a bottom wall of the steamgenerating chamber and is arranged to have a high point.

The water inlet valve is opened and closed in accord with a duty cyclewhich alternates between two phases: a fill phase duty cycle and asteady state duty cycle. The steady state duty cycle is relativelyslower than the fill phase duty cycle such that during the fill phasewater the level of water in the steam generating chamber rises and inthe steady state phase the level of water in the steam generatingchamber decreases.

In a boil dry condition (over threshold), the high point of the heaterplate begins to overheat first such that a temperature sensor mounted tothe heater plate at the high end of the heater plate can operate toprevent overheating of the heater plate. It is also possible to sensefor leakage current to allow for safe operation of the steam generatorat critical heat flux, thus ensuring a high efficiency and fast steamgeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the accompanyingdrawings, which are provided by way of non-limiting example and inwhich:

FIG. 1 is a perspective view of a cooking apparatus having a steamgenerator embodying the principles of the present invention.

FIG. 2. is a schematic illustration of the cooking apparatus showing themajor components of the cooking apparatus, including a steam generatoraccording to the present invention.

FIG. 3. is a schematic, cross-sectional view of the steam generator.

FIGS. 4 and 5 illustrate schematically alternate embodiments of theheater plate of the present invention.

FIG. 6 a is a graph showing the pool boiling curve for water atatmospheric pressure.

FIG. 6 b is a schematic illustration of the various stages of the poolboiling curve.

FIG. 7 is a graph showing the temperature control of the steam generatorheater plate in accordance with the present invention.

FIG. 8 is a schematic illustration of the inputs and outputs of thecontroller of the present invention.

FIG. 9 is a flow chart illustrating the operation and control of thesteam generator of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is shown a cooking apparatus having asteam generator in accordance with the present invention. The cookingapparatus includes of a bowl or sink-like cooking chamber set into anupper surface of a counter top. A lid is provided hingedly supportedabove the cooking chamber. Alternatively, the lid may be a separateelement and not hingedly connected to the cooking chamber. The cookingchamber is designed to receive various types of cooking utensils, suchas a perforated basket (not shown). A user interface which can include aset of controls and indicator lamps, is provided for the cookingapparatus. These controls or user interface may be preferably locatedalong the peripheral edge of the cooking chamber although they can alsobe separated from the body of the cooking apparatus. The cookingapparatus may be a “stand alone” device or may be incorporated into acook top or cooking hob with conventional burners.

FIG. 2 shows various elements of the cooking apparatus 10 in greaterdetail. It should be understood that FIG. 2 is schematic in nature anddoes not illustrate the particular appearance or configuration that maybe used in a commercial implementation of the present invention. Thecooking chamber or sink is designed to receive water and/or steamdepending on the cooking select. A water valve is provided on a waterinlet supply to regulate the supply of water into the cooking sink.Water supplied through the water valve is directed through inlet tubesto inlet housings which are provided along the top, upper edge of the ofthe cooking sink. Water flows into the cooking sink through the inletports located along the upper edge of the cooking sink.

A heating element is provided along the bottom surface of the steamgenerator. After water is added into the cooking chamber, the heater canbe used to heat the water for cooking food items placed in the cookingchamber. Alternatively, food items can be added into the cooking chamberand heated directly by the heating element such that the cooking chamberfunctions as a pan to braise or brown food items. A drain pipe extendsfrom the bottom of the cooking chamber for draining water from thechamber. A valve is provided for selectively controlling the draining.

It is also possible to add other heating type systems to the cookingapparatus. For example, heating lamps (not shown) may be provided alongthe upper edge of the cooking chamber or along the lid to supply radiantenergy into the cooking chamber. Alternatively, it is possible to directMW energy into the cooking chamber.

A steam generator is also provided for introducing steam into thecooking apparatus 10 in accordance with the invention. Water is suppliedto the steam generator through the water inlet supply which iscontrolled by a water valve. A steam conduit provides a path for steamto flow from the steam generator to the cooking chamber. The steamconduit may be connected to the inlet housing such that steam isintroduced from the chamber via the inlet ports.

Turning to FIG. 3, the steam generator of the present invention is shownschematically. The steam generator includes a casing or housing which ispreferably a plastic material with a low thermal mass. The housingdefines a steam generation cavity or chamber. A heater plate forms abottom wall of the steam generation cavity.

The heater plate is composed by a metallic plate on which a thick filmheating element is deposited upon a substrate layer of ceramic material.The face of the metallic plate opposite the thick film heating elementis directed toward the inner space or steam generation cavity. The useof a thick film heating element allows for very quick heating as thethermal mass of the heater plate is relatively small. Very fast waterheating is also achieved because the thick film heater allows for inputof a large amount of energy into the cooking chamber. Finally, thetemperature of the thick film heater can be precisely controlled.

The heater plate is arranged to have a high point. As shown in FIG. 3,the heater plate may be mounted in such a way that it is placed on aplane P that is at an angle from the horizontal orientation H. In thisway, the heater plate has a high end. The heater plate is shown orientedat an angle via the configuration of the housing. However, this may beaccomplished in any number of ways including through the use of mountingfeet extending from the housing or through the use of a speciallyconfigured metallic plate. The present invention is directed to coverany method used for orientating the heater plate at an angle.

Alternatively, the heater plate may be arranged in other ways to achievehaving a high point. In FIGS. 4 and 5, two alternate embodiments of theheater plate is shown. In FIG. 4, the high point 63′ is provided byforming a small upward protrusion or dimple in the heater plate 56′. InFIG. 5, the high point 63″ is provided by bowing upwardly the middleportion of the heater plate 56″.

Returning to FIG. 3, it is seen that provided within the housing is aninlet water opening through which water is added or sprayed into thesteam cavity. The inlet opening is fluidly connected to the water valvefor selectively controlling water inlet into the steam cavity. Afterwater enters the steam cavity, heat from the heater plate operates toheat water and convert water to steam. Steam then exits the steamgenerator through steam outlet port or opening. The steam passes throughthe steam conduits to the cooking chamber.

A temperature sensor is mounted on the underside of the heater plate atthe high point on the heater plate. In this way, the temperature sensoris positioned to sense when the heater plate begins to overheat. As longas sufficient water is present, the heater plate will not overheat.Overheating of the heater plate will occur when the steam generatorbegins to run dry. Since the highest part of the heater plate will bethe first to run dry, the temperature sensor will be able to sense whenthe heater plate first begins to overheat. A similar temperature sensor64′ and 64″ can be provided in the alternate embodiments shown in FIGS.4 and 5.

The inclusion and location of sensor is designed to improve thelikelihood of detecting a temperature rise above the critical heat fluxpoint, but operates only at location. It is also possible to measure thecurrent leakage between the heater element (resistive track) and steelplate to measure the critical heat flux temperature over the entiresurface of heater plate. Current leakage will occur, over apredetermined minimum threshold, only in the event of a hightemperature, over threshold condition, such that current leakage is agood measure of over-threshold temperature condition. In this way, twodifferent types of over threshold sensing system can be employed.

Two key performance measures for the steam generator 40 are: 1) that itproduce steam very fast; and 2) that it produce steam efficiently. Inorder to achieve these results the present invention provides a systemto control the temperature and heat transfer of the heater plate 56, theamount of water in contact with the heater plate, and the speed at whichthe heating plate heats. Moreover, the inventors have discovered that inorder for the steam generator of the present invention to be capable ofrapidly producing steam, it is beneficial to have a large surface areawhich is completely covered by water with a heater plate that is attemperature close to critical heat flux.

Heat flux in this case of steam generation is typically the amount ofheat, transferred to a liquid, per unit of time through a unit area.Generally, one may say that the critical heat flux is the conditionwhere the steam generation rate approaches the most efficient possible.Boiling heat transfer is a convective process which involves a change ofphase from liquid to vapour. There are two basic types of boiling: poolboiling and flow boiling. In general there appear to be three distinctregimes in pool boiling, these are the free convective evaporationregime, the nucleate boiling regime, and the film boiling regime. Theheat transfer characteristics vary drastically from one regime toanother. Nucleate boiling has a great practical value for fast steamgeneration because of the relatively high heat transfer rates. As theheating surface temperature is increased, vaporization will continue butbubbles will form on the heating surface at nucleation sites. As moreand more bubbles break away from the heating surface, the liquid nearbyis greatly agitated, resulting in a large increase of the heat transferrate. However the heat transfer rate cannot be increased indefinitelysince at some stage there will be so many bubbles covering the heatedsurface which will prevent the liquid from reaching the heated surface.Consequently there is a danger of burnout or “boiling crisis”. The peakheat flux for nucleate boiling at this point is called the critical heatflux. Typically this point of “critical heat flux” occurs below theLeidenfrost point, or minimum surface temperature to support filmboiling, since for film boiling the heated surface is covered with alayer of vapour preventing contact between the liquid and the heatedsurface. FIGS. 6 a and 6 b illustrate pool boiling curve and provideimages of various stages in it, respectively.

Accordingly, in order to maximize speed and efficiency, it is desirableto control the temperature of the heater plate close to the criticalheat flux temperature but not exceeding it. The critical heat fluxtemperature is dependent on atmospheric conditions as well as thecharacteristics of the water used, however it is approximately between110° C. and 140° C., but could be higher or lower depending onenvironmental conditions and water characteristics.

To promote rapid water heating, especially during initial conditions itis also helpful to have only a small volume of water in contact with theheater plate. To achieve this end, it is necessary to arrange the heaterplate in such a way that only a small volume of water is sufficient tocompletely cover the heater plate, including the high point. In FIG. 3,it can be understood that it is important to have the angle A at whichthe heater plate is supported be relatively shallow. A steep angle, forexample, 20° off horizontal, will lead to a relatively deep covering ofwater over the heater plate on the low end, contributing to a relativelylarge volume of water. Preferably, the heater plate is supported at anangle A from approximately just slightly greater than 0 to 10° offhorizontal, in the case where the heater plate is inclined. In the casewhere the heater plate is configured in some alternate way to provide ahigh point, it is also necessary to minimize the amount of total waterabove the heater plate.

The speed at which the heater plate is heated is a function of heatertype. In the present invention, the thick film heating element providesfor inputting a large amount of energy and extremely rapid heatingresponse due to the low thermal mass of the heater plate. In FIG. 7, itcan be seen that the heater plate having the thick film heating elementis capable of heating to just below the critical heat flux temperaturevery rapidly and then entering a steady state temperature.

Turning now to FIG. 3 and FIG. 8, it can be understood that the controlsystem for the present invention includes a controller or controlcircuit which analyzes temperature measurement signals from thetemperature sensor, leakage current and other sensors and controls anelectric switch in response. The electric switch is arranged between apower supply and the heating element. If the control circuit senses thatthe heater plate is overheating, the supply of power to the heatingelement can be controlled, as well as flow rate of water enteringchamber.

The controller is able to produce in a short period of time (flashboiling) a first amount of steam perceived by the consumer and in thesecond time guarantee precisely the amount of steam required by theconsumer. Looking at FIG. 7, it can be seen that the controller hasinputs from 1) the temperature of the sensor placed on the bottom of thethick-film heater on line and the user desired rate of steam on line.The controller may also receive input regarding the leakage current asdiscussed above. The controller outputs include the heater element dutycycle on line and the water valve duty cycle on line.

The supply of water into the steam generator can be separated into twomain phases: the fill phase (flash boiling), and the steady-state phase.The fill phase is initiated upon start up of steam production to createa water film on the surface of the heater plate. During this phase, thecontroller outputs a water valve duty cycle which pulses the water valveON and OFF to rapidly provide this water film on the surface of theheater plate. During the fill phase, the controller also outputs a dutycycle to the heater to rapidly raise the temperature of the heater plateto close to the critical heat flux for the water and is regulated forexample by a PID temperature controller.

The steady-state phase starts after the fill phase with a regulationcontrol of the steam flow. The correlation between the amount of steamrequired by the consumer and the heater temperature surface pulse theON/OFF switch on the inlet water valve. The steam flow regulation can besimple look-up tables or a more sophisticated closed loop system like aPID. During this phase, if leakage current is sensed or the temperaturesensor indicates an over threshold condition, the inlet water valve canbe cycled for a period of time at the higher rate employed during thefill phase to create again a water film on the entire surface of theheater.

The above description can be illustrated in a flow chart as shown inFIG. 9, where the fill phase (flash boiling) is described in steps.After a steam generation operation is initiated, as shown in step, atimer which can be part of the control circuit 70, is set to zero(TIME=0), the water valve and heating element are not energized. In step82, the timer is initiated and the water valve is pulsed to supply waterinto the steam cavity. The pulse rate for the water valve is MAX_FLOW.Water is inlet in the steam cavity for a predetermined time as shown insteps 84 and 86.

After an initial time FillPhase_Time, the controller initiates heatingthe heater plate by pulsing the heating element. The controller outputsa duty cycle to the heater to rapidly raise the temperature of theheater plate to close to the critical temperature for the water and isregulated for example by a PID temperature controller.

After a predetermined time, Fillphase=EndTime, the flow rate of thewater inlet valve is reduced to a USER_FLOW rate for a predeterminedtime as shown in step 94. This period is the steady-state phase referredto above. This steady-state phase continues for the period selected bythe user and, as described above, the amount of steam required drivesthe duty rate of the water valve and the duty rate of the heater.

As shown in steps 96 and 98, it can be seen that during the steady stateoperation, the temperature of the heater plate is monitored and in theevent of an over-threshold temperature, an action is taken, such asincreasing the rate of pulsing the water inlet valve. In particular, ifleakage current is sensed or the temperature sensor indicates an overthreshold condition, the inlet water valve can be cycled for a period oftime at the higher MAX_FLOW employed during the flash-boiling phase tocreate again a water film on the entire surface of the heater.

As discussed above, the controller outputs a duty cycle to control theheating element. This duty cycle rate may be energized in accord with auser input to provide different levels of heat energy input. In thisway, it is possible to control the amount of steam produced by the steamgenerator. To produce large quantities of steam, the heater is cycled ata high duty rate while a low duty cycle will produce less steam.

It can be seen, therefore, that the steam generator is able to operatesafely with a relatively small amount of water provided above the heaterplate by switching the duty cycle of the water valve between a fillphase and a steady state phase. This type of system allow water inletcontrol without the need for expensive and complicated control systemsfor monitoring the presence of water. This method of operation alsoprovides for very fast steam generation. Due to the relatively shallowpool of water provided over the heater plate and high efficiency, thewater heats rapidly and quickly is converted to steam.

While the steam generator is shown as part of a cooking apparatus in thepresent application, it can be appreciated that the steam generator ofthe present invention may be used in any number of applications. Forexample, the steam generator may be used with a conventional ormicro-wave oven.

Additionally, while the steam generator has been shown as a “standalone” type device, the steam generator of the present invention mayalso be incorporated as an integral part of a cooking apparatus or ovensystem. In such an integrated system, the controller of the steamgenerator may be incorporated as part of the controller for a cookingapparatus. For example, the controller may be formed as part of the userinterface control.

As is apparent from the foregoing specification, the invention issusceptible of being embodied with various alterations and modificationswhich may differ particularly from those that have been described in thepreceding specification and description. It should be understood that wewish to embody within the scope of the patent warranted hereon all suchmodifications as reasonably and properly come within the scope of ourcontribution to the art.

1. A steam generator for producing steam, comprising: a housing isprovided for forming a steam generating chamber, the housing having asteam outlet port; a heater plate is supported by the housing within thesteam generating chamber energized to a temperature near, but notexceeding, the critical heat flux temperature; and a water inlet valvefluidly connected to the housing for selectively supplying water intothe steam generating chamber whereby water is directed to the heaterplate for conversion to steam.
 2. The steam generator as claimed inclaim 1, wherein the heater plate forms a bottom wall of the steamgenerating chamber and includes a metallic plate which supports a thickfilm heating element.
 3. The steam generator as claimed in claim 2,wherein the thick film heating element is cycled at a plurality ofdifferent duty cycles for generating different levels of heat energyoutput so that the amount of steam produced by the steam generator maybe controlled.
 4. The steam generator as claimed in claim 1, wherein theheater plate includes a metallic plate which supports a thick filmheating element, and a controller is provided for outputting a dutycycle to the heating element and using feedback data such that theheating element rapidly raises the temperature of the heater plate to atemperature near, but not exceeding, the critical heat flux.
 5. Thesteam generator as claimed in claim 1, wherein the heater plate forms abottom wall of the steam generating chamber.
 6. The steam generator asclaimed in claim 5, wherein the heater plate is configured to have ahigh point.
 7. The steam generator as claimed in claim 5, wherein theheater plate is supported at an inclined angle from horizontal.
 8. Thesteam generator as claimed in claim 7, wherein the heater plate issupported at an inclined angle of between 0 and 20 degrees.
 9. The steamgenerator as claimed in claim 6, wherein a temperature sensor is mountedto the heater plate at the high point.
 10. The steam generator asclaimed in claim 1, wherein the water inlet valve is opened and closedin accord with a duty cycle which alternates between two phases.
 11. Thesteam generator as claimed in claim 1, wherein the water inlet valve isopened and closed in accord with a duty cycle which alternates between afill phase duty cycle and a steady state duty cycle, wherein the steadystate duty cycle is relatively slower than the fill phase duty cyclesuch that during the fill phase water the level of water in the steamgenerating chamber rises and in the steady state phase the level ofwater in the steam generating chamber decreases.
 12. A method foroperating a cooking appliance having a steam generator having a housingwhich is provided for forming a steam generating chamber, the cookingappliance having a controller, comprising: a water inlet valve supplyingwater into the steam generating chamber is pulsed in accord with a firstduty cycle in order to provide a film of water over a heater platesupported by the housing within the steam generating chamber, the heaterplate having a thick film heating element, the heating element isenergized to rapidly raise the temperature of the heater plate to atemperature near, but not exceeding, the critical heat flux temperature,after a predetermined time, the water inlet valve is pulsed in accordwith a second duty cycle, which is relatively slower than the first dutycycle, to provide a steady state of operation for the steam generator.13. The method of operating the cooking appliance as claimed in claim12, wherein during the steady state operation the temperature of theheater plate is monitored and in the event of an over-thresholdtemperature, the rate of pulsing the water inlet valve is increased. 14.The method of operating the cooking appliance as claimed in claim 12,wherein during the steady state operation the temperature of the heaterplate is monitored and in the event of an over-threshold temperature,the duty cycle of the heating element is changed.
 15. The method ofoperating the cooking appliance as claimed in claim 12, wherein thecontroller receives signals from a temperature sensor located at a highpoint on the heater plate and changes the control output in the event ofan over-threshold temperature.
 16. The method of operating the cookingappliance as claimed in claim 15, wherein the controller receives asignal indicative of current leakage between the heating element and ametallic plate upon which the heating element is mounted and changes thecontrol output in the event of a signal indicating current leakage.